The present invention relates generally to image-forming devices, and more particularly to a development device for use with the electrophotographic image-forming device. The present invention is suitable, for example, for a development device (including a development roller, a blade, and a reset roller) using a nonmagnetic monocomponent developing agent, and an electrophotographic image-forming device having the development device. However, it is to be understood that the scope of application of the present invention is not limited to devices using the nonmagnetic monocomponent developing agent.
The xe2x80x9cnonmagnetic monocomponent developing agentxe2x80x9d is a single component developing agent that is not magnetized and includes no carrier. The xe2x80x9celectrophotographic image-forming devicexe2x80x9d is an image-forming device employing the Carlson process described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact printer that provides recording by depositing a developing agent as a recording material on a recordable medium (e.g., printing paper, and OHP film).
With the recent development of office automation, the use of electrophotographic image-forming devices such as a laser printer for computer""s output devices, facsimile units, photocopiers, etc. has spread steadily. Particularly, an electrophotographic printer as an example of the electrophotographic image-forming devices features good operability, usability for a wide range of media, high cost efficiency, and high printing quality, whereby a further improvement in high-quality and high-speed printability will be expected in future. The electrophotographic image-forming device generally includes a photoconductive insulator (photosensitive drum), and follows the procedural steps of charging, exposure to light, development, transfer, fixing, and other post-processes.
The charging step uniformly electrifies the photosensitive drum (e.g., at xe2x88x92780V). The exposure step irradiates a laser beam or the like on the photosensitive drum, and changes the electrical potential at the irradiated area down, for example, to xe2x88x9260 V or so, forming an electrostatic latent image. The development step electrically deposits a developing agent onto the photosensitive drum using, for example, the reversal process, and visualizes the electrostatic latent image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developing agent having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer step forms a toner image corresponding to the electrostatic latent image on a recordable medium. The fixing step fuses and fixes the toner image on the medium using heat, pressure or the like, thereby obtaining a printed output. The post-processes may include charge neutralization and cleaning on the photosensitive drum from which toner has been transferred out, a collection and recycle and/or disposal of residual toner, etc.
The developing agent for use with the aforementioned development step can be broadly divided into a monocomponent developing agent using toner and a dual-component developing agent using toner and a carrier. The toner may include a particle prepared, for example, in such a manner that a colorant such as a dye and a carbon black, or the like is dispersed in a binder resin made of synthetic macromolecular compound, and then is ground into a fine powder of approximately 3 through 15 xcexcm. A usable carrier may include, for example, an iron powder or ferrite bead of approximately 100 xcexcm in diameter. The monocomponent developing agent advantageously results in: (1) simple and miniature equipment of the development device due to eliminating a carrier deterioration, a toner density control, mixing, and agitation mechanisms; and (2) no residual waste such as a carrier in used toner.
The monocomponent developing agent may be further classified into a magnetic monocomponent developing agent that includes a magnetic powder in toner, and a nonmagnetic monocomponent developing agent that does not include the same. However, the magnetic monocomponent developing agent is disadvantageous in: (1) the low transfer performance due to the high content of low electrically resistant magnetic powder which hinders the increased electric charge amount; (2) the bad colorization due to its low transparent, black-color magnetic powder; (3) the low fixing performance due to the magnetic powder which requires high temperature and/or high pressure, increasing a running cost. Accordingly, the nonmagnetic monocomponent developing agent without these disadvantages is expected to be in increasing demand in future.
The nonmagnetic monocomponent developing agent commonly includes the toner having a relatively high volume resistivity (e.g., at 300 Gxcexa9xc2x7cm, etc.). In addition, the toner, as basically carries no electric charge, needs to be charged by the triboelectrificity or charge injection in the development device.
The development process employing the nonmagnetic monocomponent developing agent is divided into contact- and noncontact-type development processes: the contact-type development process that deposits a developing agent on the photosensitive drum by bringing the development roller carrying the developing agent into contact with the photosensitive drum; and the noncontact-type development process that provides a certain gap (e.g., of about 350 xcexcm) between the development roller and the photosensitive drum to space them from each other, and flies the developing agent from the development roller to and deposits the same onto the photosensitive drum.
It is significant for the noncontact-type development process employing the nonmagnetic monocomponent developing agent to ensure a sufficient image density by controlling the amount of toner conveyed from the development roller to the photosensitive drum. Thus, it is very important to form a specified toner layer while controlling its thickness on the development roller. As a typical method for regulating a toner layer thickness, it has conventionally been proposed to provide a blade (restriction blade) in contact with the development roller to maintain the layer thickness uniform.
The noncontact-type development device employing the nonmagnetic monocomponent developing agent comprises a toner tank, an agitation paddle, a development roller, a reset roller, and a blade. The toner tank is configured to store toner required for printing, and to supply toner to the reset roller. The agitation paddle is provided in the toner tank, and serves to prevent the toner from agglomerating and coagulating by agitating the toner. The development roller adsorbs onto a surface thereof charged toner in the form of a thin layer, and conveys the toner to a development area in contact with the photosensitive drum. The development roller is connected with a bias power supply that applies a development bias. The reset roller, which is also called supply roller or application roller, contacts the development roller and supplies toner to the development roller. Further a reset bias is applied to the reset roller. The reset roller also serves to scrape off and remove the toner unused for the development and remaining on the development roller, and to collect the same to the toner tank for recycling.
The blade is brought into contact with the development roller, and serves to regulate the toner layer to a uniform thickness. A blade bias is applied to the blade, making the charge injection into toner possible. A toner layer on the development roller, if too thin, would result in a low and uneven image density, while, if too thick, would increase a proportion of oppositely charged or low charged toner, thereby producing a fog in a no-image area (i.e., undesirably coloring with the toner an area which has no image and is therefore expected to be white clarity). Thus, the blade is required to form a toner layer having an appropriate thickness.
In development operation, the toner is charged (e.g., negatively) through sliding friction among the reset roller, the blade, and the development roller. The negatively charged toner is thereafter fed onto a surface of the development roller by the reset roller, and deposited thereon by electrostatic adsorption. Subsequently, the toner layer on the development roller is leveled using the blade to form a thin layer having a uniform thickness of about 10 xcexcm. The toner, which has been conveyed to a development area, gets adhered to an electrostatic latent image on the photosensitive drum with the electrical force of attraction using a predetermined voltage applied to the development area. Consequently, the latent image is visualized and developed. Next, the residual toner unused for the development is scraped off and removed by the reset roller from the development roller, and collected into the toner tank. The development process repeats a series of these operations.
In the above-described development process, the toner is stressed in various ways, which produces deterioration of the toner. The toner deterioration means a deterioration in a surface condition due to separation or embedment of external additives for adding charge and fluidity, and, in addition, an increase of powdered toner due to pulverization of the toner. In the development operation, the toner is stressed by: (1) the sliding friction among the reset roller, the blade, and the development roller; (2) the contact with the photosensitive drum and the development roller; and (1) the agitation stress in the toner tank caused by the agitation paddle. Particularly, a large proportion of the residual toner unused for the development and left on the development roller includes such deteriorated toner, and repeated use of the toner for printing would further impose the stress on the same toner, whereby the deterioration of the toner progresses steadily. The deterioration of the toner may be proved, for example, by measuring a lowered fluidity and increased proportion of powered toner.
As the deterioration of the toner progresses, a charging property of the toner lowers. This would resultantly increase a proportion of low charged or oppositely charged toner in the toner for use in the development, and increase susceptibility to fogging in the no-image area. Therefore, it has been conventionally proposed to provide a method of preventing the charging property of the toner from lowering by injecting charge from outside into the low charged toner in order to repress or reduce fogging. It has also been proposed to provide a method of avoiding reverse charge injection into toner in contact with the development roller by setting electrical resistance of the development roller at a substantially high level, thereby preventing the low charged or oppositely charged toner from being produced.
However, the conventional method of repressing disadvantageous fogging in the contact-type development process using a nonmagnetic monocomponent developing agent has detrimental effects such as lowering development efficiency and producing image retention in an early stage of printing. Particularly, if the resistance of the development roller were set at 1010xcexa9 or higher, charges would accumulate on a surface of the development roller, and vary the development voltage, thereby producing a fog and lowering the development efficiency. The image retention is a phenomenon in which the image density partially varies under the influence of an image previously printed.
The development efficiency lowers, because a charge amount of toner increases, and a total charge amount of the toner layer becomes higher than the total charge amount (Q=CV) estimated utilizing a voltage applied when a photosensitive drum-toner layer-development roller relationship is deemed to work as a capacitor. In this case, the toner on the development roller would not be sufficiently utilized for the development, lowering the image density.
In the aforementioned development operation, part of the toner on the development roller corresponding to an image is adsorbed onto the photosensitive drum, and the other part of the toner corresponding to a no-image area is left on the development roller. The development device is configured to flake off such residual toner from the development roller by the reset roller, but actually, fails to remove a considerable amount of the toner, which is conveyed to and brought into contact with the blade once again, thereby requiring further charge injection. Accordingly, the charge amount of the toner increases, and the toner gets adhered more strongly onto the development roller by an image force. Moreover, high charged toner having a small particle diameter tends to be adhered selectively onto the no-image area on the development roller, because powdered toner has so high mechanical adherence that the reset roller cannot easily flake off the same. Thus, variation in the particle diameter and charge amount may disadvantageously result between the toner layer adhered onto an area on which toner was deposited in the next previous printing cycle and that adhered onto no-image area on which no toner was deposited then. Consequently, a high-density image may be formed in an area corresponding to the toner-deposited area on an immediately preceding printed page, while a low-density image may be formed in an area corresponding to the no-image area on an immediately preceding printed page. This phenomenon is called positive image retention. Particularly, the image retention is caused by an increase of the toner charge amount, and thus frequently produced when the toner is substantially high charged, as in a low-humidity environment or in an early stage of printing.
In order to repress the occurrence of the image retention, it may be a conceivable measure to add a neutralizing means for eliminating charge in residual (undeveloped) toner on the development roller. However, this measure would require an additional instrument provided in the development device, and disadvantageously increase the complexity and costs of the device.
Moreover, the resistance of the development roller set at 105xcexa9 or lower could serve to repress the occurrence of the positive image retention in an early stage of printing, but if the device were operated in a high-humidity environment, and the toner were deteriorated, the charge would be injected from the development roller to the toner, and make a fog liable to be produced.
Further, as a means for repressing both of the positive image retention and fogging, it may be a conceivable measure to adjust voltages applied to the development roller and reset roller according to the deterioration of the toner due to repeated printing operations. To be more specific, this measure facilitates collecting the toner on the development roller by shifting a voltage difference applied to the development roller and the reset roller into preferable polarity for collecting with respect to toner charging polarity. This measure would improve a toner collecting capability of the reset roller, and thus reduce the occurrence of the positive image retention. However, this measure would reduce a toner supply to the development roller by the reset roller, and make the reset roller likely to collect the toner having a high charge amount, thereby disadvantageously making the device liable to produce negative image retention. In addition, in order to adjust and control the applied voltages in such a manner, a complicated power unit or sequencer need be provided, leading to an increase in costs.
Accordingly, it is an exemplified general object of the present invention to provide a novel and useful development roller and blade used in a development device, development device and image-forming device having the development roller and the blade, in which one or more of the above-described conventional disadvantages are eliminated.
Another exemplified and more specific object of the present invention is to provide a development roller and blade used in a development device, and development device and image-forming device having the development roller and the blade that may cost-efficiently form a high-quality image.
In order to achieve the above objects, the development roller as one exemplified embodiment of the present invention comprises a shaft, a base material that coats a periphery of the shaft, and is made of a resistance body, and a layered coating provided on a periphery of the base material, wherein the layered coating includes two or more layers, and a coating layer provided inside an outermost coating layer has a higher charging capability to a developing agent than at least the outermost coating layer. According to the development roller, even if repeated printing operations scraped away the outermost coating layer and deteriorated toner, the coating layer provided inside with a higher charging capability than the outermost coating layer would be exposed, and thus could prevent the charging capability of toner from lowering.
The blade as one exemplified embodiment of the present invention comprises a base material made of metal and shaped like a plate, and a layered coating provided on one surface of the base material, the surface being in contact with a development roller, wherein the layered coating includes two or more layers, and a coating layer provided at least inside an outermost coating layer has a higher charging capability to a developing agent than the outermost coating layer. This blade has a layer structure similar to the above development roller, and thus can prevent the charging capability of toner from lowering due to repeated printing operations.
The development device as one exemplified embodiment of the present invention comprises a development roller, and a blade that is brought into contact with the development roller, and forms a toner layer at a predetermined layer thickness, wherein the development roller comprises a shaft, a base material that coats a periphery of the shaft, and is made of a resistance body, and, a layered coating provided on a periphery of the base material, wherein the layered coating includes two or more layers, and a coating layer provided at least inside an outermost coating layer has a higher charging capability to a developing agent than the outermost coating layer. This development device includes the above development roller, and thus can prevent the charging capability of toner from lowering due to repeated printing operations. Accordingly, fogging due to the repeated printing operations may be prevented. In addition, in an early stage of a printing process, low charging capability to the toner is provided, and thus an upsurge in toner charge amount beyond the amount as deemed necessary can be restricted, and image retention may also be prevented.
The development device as another exemplified embodiment of the present invention comprises a development roller, and a blade that is brought into contact with the development roller, and forms a toner layer at a predetermined layer thickness, wherein the blade comprises a base material made of metal and shaped like a plate, and a layered coating provided on one surface of the base material, the surface being in contact with a development roller, wherein the layered coating includes two or more layers, and a coating layer provided at least inside an outermost coating layer has a higher charging capability to a developing agent than the outermost coating layer. The development device includes the above blade, and thus can prevent the charging capability of toner from lowering due to repeated printing operations. Accordingly, fogging due to the repeated printing operations may be prevented. In addition, in an early stage of a printing process, low charging capability to the toner is provided, and thus an upsurge in toner charge amount beyond the amount as deemed necessary can be restricted, and image retention may also be prevented.
The image-forming device as one exemplified embodiment of the present invention comprises a photosensitive body, a charger that charges the photosensitive body, an exposure section that exposes the photosensitive body charged by the charger to light, and forms an electrostatic latent image, a development device that develops the photosensitive body exposed to light, and visualizes the electrostatic latent image into a toner image, and a transfer section that transfers the toner image onto a recordable medium, wherein the development device comprises a development roller and a blade that is brought into contact with the development roller, and forms a toner layer at a predetermined layer thickness, wherein the development roller comprises a shaft, a base material that coats a periphery of the shaft, and is made of a resistance body, and a layered coating provided on a periphery of the base material, wherein the layered coating includes two or more layers, and a coating layer provided at least inside an outermost coating layer has a higher charging capability to a developing agent than the outermost coating layer. The image-forming device includes the above development device, and thus has an operation similar to the development device.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.